A Wet Soil Order in Soil TaxonomyICOMSASMark Stolt

In 2015 the Soil Science Society of America established a task force to develop

fundamental changes in Soil Taxonomy.

Task Force Objective: make fundamental changes to Soil Taxonomy so that the system can be more efficient and useful to the soil science community it serves, and serve as an effective and engaging tool for the soil science community to reach other disciplines and communities that use soils information.

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Increase in Inceptisolsubgroups between 1975 and 2010

Keys to Soil Taxonomy1996 1983

Complicated

Complicated

“garbage can” taxa such as Dystrudepts and Haplustalfs

Soil Taxonomy Issues Addressed by the Task ForceRedefining kandic horizons – completed and accepted

Redefining the mollic epipedon – a proposal is in review

Redefining organic soil materials -- a proposal is in review

Moving soil moisture regime to the family level in selected taxa – a paper outlining the pros and cons of making such a change is currently in review

Creating a soil order for anthropogenic soils – not a charge for the task force but a consequence of our efforts

Creating a wet soil order for mineral soils – a proposal is in the works reported on here

Mark Stolt, Univ. of Rhode Island (chair)

Brian Needelman, Univ. of Maryland (co-chair)

Dylan Beaudette, NRCSPatrick Drohan, Penn

StateJohn Galbraith, Virginia

Tech

David Lindbo, NRCSCurtis Monger, NRCSAnthony O'Geen, Univ. of California-DavisMarty Rabenhorst, Univ. of MarylandMickey Ransom, Kansas StateJoey Shaw, Auburn

SSSA Task Force Members

Should there be a “Wet” Soil Order in Soil Taxonomy?Martin Rabenhorst1, Mark Stolt2, David Lindbo3, Curtis Monger4

1-Department of Environmental Science and Technology, University of Maryland2-Department of Natural Resources Science, University of Rhode Island

3-Soil Science Division, USDA-NRCS, Washington, DC; 4-National Soil Survey Center, USDA-NRCS, Lincoln, NE

Contact: Martin Rabenhorst mrabenho@umd.edu

ABSTRACTEarly soil classification systems recognized wet soils at thehighest categorical level. Among the Intrazonal Soils of the USclassification utilized between the 1920s and 1960, wereincluded as Great Soil Groups, the Wiesenboden, Bog, Half-Bog, Ground-Water Podzols and Ground-Water Laterites. Inother systems, groups named with such terms as groundwater gley and pseudogley were also used. With the adventof Soil Taxonomy and it’s precursor (1960, 1975), Histosols(organic soils) were distinguished as one of the initial 10 soilorders, and while many of these organic soils are wet soils,some are not (Folists for example). Thus, for over 50 years,with the exception of Histosols, wet soils (which typicallyrepresent the wettest end of subaerial wet soils) have notbeen collectively recognized within taxa at the highestcategorical level (order) in the US soil classification system.Rather, the wettest soils were designated at the secondcategorical level as wet (Aqu) suborders among the varioussoil orders, and more recently, subaqueous soils as “Wass"suborders of Entisols and Histosols. Soils with less-wetconditions have been recognized at the subgroup (4th) level.Further, in impoundments and regions of transgressingcoastlines, submerged upland soils have been found that stillclassify in soil orders that do not accommodate subaqueoussoils (“Wass" suborders). Notwithstanding, othercontemporary soil classification systems do (have continuedto) recognize wet soils at the highest level. In the WorldReference Base (WRB) for example, wet soils are designatedas Gleysols or Stagnosols. As efforts are underway to revisit,simplify, and revise Soil Taxonomy, questions have beenraised regarding whether wet soils should again be movedback with a place among taxa at the highest category using aname such as Hydrasols, Aquasols, etc. This paper willexplore and consider the questions and arguments for andagainst such proposals and the difficult question regardingwhere along the soil wetness continuum would be the bestpoint for recognizing a wet soil order.

EARLY U.S. SOIL CLASSIFICATION SYSTEM SOME ARGUMENTS FOR ESTABLISHING A WET SOIL ORDER • The National Soil Survey Handbook (Soil Survey Staff, 2015) recognizes over 30 interpretations that are driven by the depth of

the water table. Therefore, establishing a wet soil order would permit recognition of one of the most important properties governing interpretations (extreme or extended periods of soil wetness) at the highest categorical level.

• When Soil Taxonomy was developed over 50 years ago, hydric soils and wetlands were considered of low economic and environmental importance, but in the intervening decades they have become recognized as providing valuable ecosystem services. Therefore, establishing a wet soil order could help in developing a better correspondence between Soil Taxonomy and hydric soils, although we understand that specific morphological indicators for hydric soils will continue to be developed regionally.

• The WRB recognizes wet soils (Gleysols and Stagnosols) at the highest level as reference soil groups. Therefore, establishing a wet soil order would be more like the WRB and would be consistent with the guiding principles to: “complement the concepts used in other soil taxonomic systems (specifically the WRB)”. This should improve “buy-in” from the international community.

• Currently, Aqu suborders are defined differently depending on the order. The complexity arising from these differences make the learning and use of Soil Taxonomy much more difficult. Therefore, establishing a wet soil order would permit application of more uniform (and simplified) criteria in the recognition of wet soils across the spectrum.

WRB 2014

Gleysols GL: Groundwater-affected soils, underwater soils and soils in tidal areasStagnosols ST: Stagnating water, structural difference and/or moderate textural difference

WORLD REFERENCE BASE (WRB) 2014

GUY SMITH’S RATIONALE IN SOIL TAXONOMYWhen asked specifically why wet soils were handled the way they were in Soil Taxonomy . . . • Acknowledgment “In Soil Taxonomy we divided up the wet soils and we put

them at the suborder level, not at the order level,” . . . .“most other taxonomies have an all wet soils group.” “the Europeans . . . want one order for all the wet soils.”

• Explanation: “There was a zonality to the soils with aquic moisture regimes and this would be best reflected if the aquic soils with aquic moisture regimes were separated below the order level.”

• “compared the yields on . . . . . the plots that were all Udolls and the plots that were all Aquolls (had been drained). . . . . they were identical.”

• “if one goes into the Southeast, in the region of Ultisols, one would have the same experience, that after drainage the naturally poorly drained soils will behave like the naturally well drained soils of that area.”

WD MWD MWD SWPD SWPD PD VPD

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Peraquic SubaqueousUpland – Wetland CatenaSoil Wetness Continuum

Boundary 1. Current Aqu suborders: Include very poorly, poorly, and at least some somewhat poorly drained soilssustained aquic conditions within 40 cm of the soil surfaceConsiderations: a. would include many soils across most orders, some of which are not especially wet (easily drained)b. Guy Smith’s argument regarding zonality of wet soils suggests keeping these within other orders

Boundary 4. Subaqueous soils: permanently flooded too deep to support emergent vegetationuse current definition for Wass suborders of EntisolsConsiderations: a. This break would separate only soils in subaqueous landscapes, a very narrow concept of the wettest soils. b. It would be the easiest to implement and would have smallest impact on present Soil Taxonomyc. A great many wet soils would not be included within this order of wet soils.

Boundary 3. Peraquic soils: essentially permanently wet, such as swamps, marshes, bogs, fens etc, supporting emergent vegetationnearly continuous aquic conditions at or near the soil surfaceConsiderations: a. This break includes only soils that have little potential for drainage or agriculture; the wettest of wet soilsb. This leaves many very wet soils outside of this class of wet soils

Boundary 2. Typic/not-Aeric portions of current Aqu suborders: exclude marginally wet soils (somewhat poorly drained)sustained aquic conditions within some shallower depth - 25 cm of the soil surface? 10 cm of the surface?Considerations: a. takes only those soils that are wettestb. Guy Smith’s argument regarding zonality of wet soils may still argue for keeping these within other orders

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Voting of 40 Individuals at SSSA 2016 Phoenix, AZ

1 - Current Aqu suborders

2 - Typic portions of Aqusuborders3 - Peraquic soils

4 - Subaqueous soils

No Wet Order

PERSPECTIVES AND OPINIONS OF US SOIL SCIENTISTSAt the 2016 meeting of SSSA in Phoenix, AZ an informal (unofficial and non-binding) poll was conducted during a poster session. Respondents were asked at which boundary point they would prefer the boundary between soils that are, and are not, included within a new wet soil order. Opinions were widely mixed. A plurality preferred boundary #3 between soils that do / do not have a peraquic moisture regime.

IV

International Committee on Subaqueous and Aquic SoilsICOMSAS

Martin Rabenhorst -- University of MarylandDavid Lindbo – USDA-NRCS SSD LeaderCurtis Monger – USDA-NRCS Soil Taxonomy LeaderJo Parsley – USDA-NRCS Soil Taxonomy Focus Group ChairJames Thompson – West Virginia University Matthew Ricker – North Carolina State UniversityPeter Schad – Technische Universität München, GermanyCornie van Huyssteen – University of the Free State, South AfricaRob Fitzpatrick – University of Adelaide, AustraliaMark Stolt – University of Rhode Island

Current1. Gelisols2. Histosols3. Spodosols4. Andisols5. Oxisols6. Vertisols7. Aridisols8. Ultisols9. Mollisols10.Alfisols11.Inceptisols12.Entisols

Proposed1. Artesols2. Gelisols3. Histosols4. Aquasols5. Spodosols6. Andisols7. Oxisols8. Vertisols9. Aridisols10.Ultisols11.Mollisols12.Alfisols13.Inceptisols14.Entisols

Keys to Soil OrdersAquasols-- formative element = aq,-- pronounced “Ack Wa Sols”

A wet soil is one that the water table comes to the soil surface and stays there long enough to severely alter the soil morphology and impact the use and management of the soils. In this proposal, we are defining this as having aquic conditions within a depth of 30 cm or less from the soil surface.

Keys to the Suborders

1) Wassaqs -- water over the soil surface continuously-- – subaqueous soils;

2) Peraqs -- water levels that stay at or near the soil surface all year long; peraquic;

3) Leptaqs -- have a root-restrictive feature within 100 cm of the soil surface

4) Vertaqs -- that have a layer 25 cm or more thick, within 100 cm of the mineral soil surface, that has either slickensides or wedge-shaped peds that have their long axes tilted 10 to 60 degrees from the horizontal

5) Humaqs – have a histic, mollic, or umbric epipedon

6) Psammaqs – have a sandy particle size class in all of the upper 100 cm of the soil

7) Argaqs – have an argillic horizon

8) Orthaqs – other Aquasols

Keys to the Wassaq Great Groups

1) Wassaqs -- water over the soil surface continuously-- – subaqueous soils;

1.1. Fluiwassaq: have fluid material that extends at least 20 cm from the soil surface

1.2. Sulfiwassaq: have sulfidic materials within 50 cm of the soil surface

1.3. Psammowassaq: texture class of loamy fine sand or coarser in all layers in upper 20 cm

1.4. Humiwassaq: histic, mollic, umbric, or melanic epipedon1.5. Haplowassaq: other Wassaqs

Keys to the Peraq Great Groups2) Peraq -- water levels within 25 cm of the soil surface at least 9 months of

the year; peraquic;

2.1. Sulfoperaq: sulfuric horizon within 50 cm of the soil surface; 2.2. Sulfiperaq: sulfidic materials within 50 cm of the soil surface; 2.3. Saliperaq: saline within 50 cm of the soil surface; 2.4. Fluiperaq: have fluid materials in the upper 30 cm;2.5. Humiperaq: histic, mollic, umbric, or melanic epipedon present;2.6. Psammoperaq: sandy textures throughout the upper 100 cm;2.7. Fluviperaq: have an irregular decrease in organic-carbon content;2.8. Haploperaq: All other Peraqs.

Keys to the Leptaq Great Groups3) Leptaqs -- have a root-restrictive feature within 100 cm of the soil surface

3.1. Petroleptaq: Leptaqs that have a cemented layer (excluding a lithic contact) within 100 cm of the soil surface;

3.2. Fragileptaq: Leptaqs that have a fragipan within 100 cm of the soil surface

3.3. Plintholeptaq: Leptaqs that have a >5 percent (v/v) plinthite within 100 cm of the soil surface;

3.4. Densileptaq: Leptaqs that have a densic contact within 100 cm of the soil surface;

3.5. Litholeptaq: Leptaqs that have a lithic contact within 100 cm of the soil surface;

3.6. Haploleptaq: All other Leptaqs that have a root-restrictive layer (e.g. paralithic contact, silcrete, alcrete, clay pan) within 100 cm of the soil surface.

DBB Sulfiperaq SubgroupsDBBA Sulfiperaqs that have, in some horizons at a depth between 20 and 50 cm below the mineral soil surface fluid, moderately fluid, or very fluid soil materials. (Broadkill)

Fluic SulfiperaqDBBB Other Sulfiperaqs that have a histic epipedon. (Matunuck)

Histic SulfiperaqDBBC Other Sulfiperaqs that have a mollic epipedon ≥60 cm thick.

Cumulic SulfiperaqDBBD Other Sulfiperaqs that have a mollic epipedon.

Humic SulfiperaqDBBE Other Sulfiperaqs that have a buried layer that meets criteria for a histic or mollic, epipedon

within 200 cm of the soil surface; or have a combined thickness of buried surfaces, A and O horizons, that is 20 cm or more thick within 200 cm of the soil surface. (Appoquinimink)

Thapto-Humic SulfiperaqDBBF Other Sulfiperaqs that have less than 35 percent coarse fragments and a texture of loamy fine sand or coarser in all layers within the particle-size control section. (Sandyhook, Demariscotta)

Psammentic Sulfiperaq

What we would like from you

Any comments on the proposalSpecifically: • have any soils been left out of the classification that you know of • are there subgroups that don’t exist but are recorded here

Send comments to me (mstolt@uri.edu) or to your Soil Taxonomy Committee chairs wayne.gabriel@usda.gov

MATTHEW.LEVI@uga.edu